Pressure relief holes needed in PML kits?

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Well-Known Member
Jan 18, 2009
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I had never heard mention of it before, but it has come up a couple of times in the past few weeks. I've seen suggestions for putting pressure relief holes below, and above the piston system. Is this a standard practice on the higher powered kits? No higher, or faster than it will go on J's and under, is it really needed?

I built this stock, with a foamed fin can. Planned motors are I's and at least one J (for a L2 attempt).
I for one have never heard of putting a pressur e vent on either side of the piston in a PML kit. Would seem to kind of negate the idea of the gas charge to ensure deployment.

You may be confusing this with the idea of vent holes to allow for accurate performance of a barometric altimeter.

If I go ouit on a limb, I guess you might put something beloe the piston if you were afraid of it jamming and then have the ejectiongas blow the airfeame apart. I've maybe 35 PML flights and never had the piston jam. I've flown the QT birds from10 degrees to 90 degrees without a problem.

Of course, I can also use a Copperhead, so my experiences may be unique.

If you're flying really high, really fast, it's a good idea. At XPRS-1, Erik Gates flew his Magneto (Glassed PML Nimbus) on a K1050W. Sounds cool. The rocket had pressure holes above the piston, in the altimeter bay, and in the main chute bay. Altimeter was an ALTACC 2C - no Mach firings here. The rocket tore off the pad, flew for about 1.5 seconds under thrust, and then shredded all over the place. Since the whole rocket was so stout (he chopped it up and uses it as a trailer hitch cover now) and the fins were definitely on after the flight, the probable cause was a pressure separation. The pressure beneath the piston was greater than above it, causing it to ride up in the airframe and push off the payload compartment. Shred city.

Just my experience. That rocket had survived previous flights on K700Ws and K1100Ts, so this was really only an extreme, extreme case. I think for a 4" rocket on anything less than an M motor, it won't be a problem.
Actually, I beg to differ. I flew my GF's PML Pteradactyl Jr. at RocStock XX last month on an H-242T and it pressure separated from the pad. Puzzled, I asked why it had happened when I had an upper vent hole, and someone suggested that I put a lower vent hole below the piston. I did so, flew the same motor again and had a beautiful flight! So I for one recommend a vent hole both above and below the piston. Of course, I think on the smaller diameter PML kits, you can get away with one above the piston so as not to separate the nose, but on larger diameter kits, definitely one above and one below.

Originally posted by SpartaChris
I think on the smaller diameter PML kits, you can get away with one above the piston so as not to separate the nose, but on larger diameter kits, definitely one above and one below.

Thinking about it again, I stand corrected. A little bit of pressure in a big diameter will go a long way in applying a force, but not so much in a smaller diameter airframe (those square inches go up fast). Hey, putting a few 1/8" holes behind the piston couldn't hurt, right?

Or I suppose you could solve the problem altogether and use shear pins ;)
Ok, I did some looking around, and found a small section in PML's online FAQ. They recommend relief holes on rockets with fast burning motors that will reach or exceed 5k pretty quickly. They go on to state that if you are unsure, or, just don't want to take any chances, to drill a 1/8" hole just above the forward most centering ring, and at the top of the main air frame below where a coupler, or nose shoulder would block it.

I think just for the sake of self sanity, I'll drill the holes as PML suggests. I've already drilled one in the payload bay, which hopefully after christmas, will be used for dual deployment :)

Thanks everyone for your input!

And, Al, I've had purty good success with copperheads my self (knocking on wood).